WASHINGTON – Advocates of aerospace firms vying to deliver cargo and crew to the International Space Station are concerned that language in a Senate spending bill a key committee passed Thursday could make it more difficult and expensive to carry out those missions.

The provision, sponsored by Republican Sen. Richard Shelby of Alabama, would require firms in the commercial crew and commercial cargo programs to submit “certified cost and pricing data” similar to what’s required in traditional contracts NASA uses for other services.

Shelby’s proposal is included in a spending bill the Senate Appropriations Committee passed 30-0 Thursday to fund several federal agencies, including NASA, in the 2015 fiscal year that begins Oct. 1.

The bill would provide NASA $17.9 billion, and include money to continue development of its major priorities, including the James Webb space telescope that will replace the Hubble, and the space launch system deep-space rocket and vehicle designed to carry astronauts to Mars by the 2030s.

The bill now heads to the full Senate and will eventually be reconciled with a similar version the House passed last week.

The Senate spending plan also includes $805 million for commercial crew, NASA’s program to replace the now-mothballed space shuttle with a small fleet of private rockets that could transport astronauts to the space station from American soil as early as 2017. Private rockets already are delivering cargo to the orbiting lab.

In an effort to keep costs down and speed development, the agency has opted to use Space Act agreements instead of traditional contracts for both crew and cargo programs.

Under the agreements, NASA pays companies to achieve certain milestones but leaves details largely to the contractor. It costs less, but the firms get to keep the intellectual property rights of their products, and there’s a risk a problem could go undetected until later in the development process.

Advocates of the arrangement say it means companies can more nimbly -- and cheaply -- meet contract targets. But skeptics like Shelby say there’s little oversight and the government has little control over costs.

“I believe we must ensure that the taxpayers are getting the best value for their dollars,” he said at an Appropriations subcommittee meeting earlier this week.

But the head of an industry trade group called Shelby’s provision “a step in the wrong direction” at a time when the U.S. is trying to end its dependence on Russia -- at $70 million a ride -- to ferry U.S. astronauts to the space station.

“The language would effectively change an efficient and lean commercial program into a traditional government procurement with all of the associated overhead and cost,” said Alex Saltman, executive director of the Commercial Spaceflight Federation.

“In addition, if this language were to become law before NASA awards the latest commercial crew contracts, NASA would likely have to restart the procurement with these new rules, pushing back the program up to a year and sending hundreds of millions of more taxpayer dollars to Russia for Soyuz rides,” Saltman added. “If the language were to go into effect after the awards, NASA could be tied up in contract renegotiations and challenges for months if not years.”

Commercial space advocates say it would cost only about $20 million for each astronaut’s seat.

NASA officials declined to comment, saying they were still reviewing the Senate language. There was no immediate comment from SpaceX. The California firm has a contract with NASA to deliver cargo to the space station and is among the competitors for the contract to transport astronauts as well.

Another commercial space advocate, the Space Access Society, said Shelby was sponsoring the language merely to protect the “massively wasteful” space launch system. Much of the work to develop SLS is being conducted at NASA’s Marshall Space Flight Center in Huntsville, Ala., which Shelby represents.

Asked about the criticism of his motives after Thursday’s hearing, Shelby said simply: “That’s not true. We’re looking for transparency.”